1. Technical Field
The invention relates to a circuit for the protection of an electrical consumer against polarity reversal, utilizing a MOSFET (metal oxide semiconductor field effect transistor), wherein the circuit is connected to the voltage supply on its input, and to the consumer on its output, and wherein the source terminal of the MOSFET is connected to the voltage supply, and the drain terminal of the MOSFET is connected to the consumer.
2. Description of Related Art
Electrical devices, electrical circuits, or electrical consumers in general which are operated with direct current are typically configured with a special protection for the purpose of protecting the electrical consumer from damage resulting from the reversed connection of the poles of the supply voltage. A diode is frequently used for this purpose. Diodes can be subjected to high currents in the forward direction, and have a high cutoff voltage. In addition, diodes are extremely cheap, such that they have little effect on the total price of the devices.
In addition, a diode in the input circuit has the decisive advantage that it creates a buffer effect together with the capacitors in the device. If the input voltage is suddenly interrupted—for example when the engine in a vehicle is started—the capacitors can maintain the functions of the device for a certain period of time by means of the charge they store. The diodes are used to prevent the charge in the capacitors from discharging in the direction of the power supply. In a vehicle, by way of example, the discharge would occur via external loads such as the starter. The diode therefore enables a certain support effect.
A decisive disadvantage of diodes is that they have a voltage loss in the forward direction of approx. 0.5 to 1 V. This voltage loss has an undesirable effect on the characteristics of the device in two ways. First, the dissipation loss of the device overall increases. As such, at a current of 2 A and 0.5 V, for example, a voltage loss of 1 W occurs in the diode, which itself has a marked effect on the self-heating of the device. Second, the operating voltage must be selected as 0.5 to 1 V higher than is actually necessary, due to the voltage loss via the diode. If the voltage reserve is not properly configured, the voltage drop via the diode can—for example in a vehicle—trigger a reset of a device upon the vehicle starting process.
To reduce the voltage loss, Schottky diodes are sometimes used in protective circuits, wherein only half of the voltage drops in this case compared to standard diodes. However, these diodes have significantly higher leakage current. In addition, the electric strength in the cutoff direction is significantly lower.
In a further protective circuit known from the prior art, MOSFETs (metal oxide semiconductor field effect transistors) are used. MOSFETs offer the advantage that the voltage drop across the transistor is only on the order of magnitude of 0.1 V. If the supply voltage is connected with the poles reversed, the MOSFET closes; if it is connected correctly, the MOSFET conducts current. As a result of the relatively small drop in voltage via the MOSFET, it is possible to greatly reduce the losses. A disadvantage of such protective circuits is that they differ significantly from a diode in their dynamic behavior. MOSFETs conduct current in both directions when connected. As a result, if an interruption occurs in the supply voltage, for example during a starting process in a vehicle, the capacitors in the device discharge via the MOSFET and via external loads. A support effect, as in the use of a diode, does not exist here.
For these reasons, the invention addresses the problem of designing and implementing a protective circuit in the class named above which protects against reverse polarity in such a manner that it is possible to realize a secure protection from reverse polarity, with a dynamic behavior similar to that of a diode, while at the same time having low dissipation loss.
According to the invention, the problem given above is addressed by the features of claim 1. According to the claim, the circuit in question is characterized in that the gate of the MOSFET is connected to the collector of a first bipolar transistor, and the source of the MOSFET is connected to the emitter of the first bipolar transistor, and in that the base of the first bipolar transistor is controlled by means of a control current, wherein the control current is tapped from the voltage at the drain of the MOSFET.